Currencies you can open an account in

The currencies `POST /virtual-accounts` can open on its own. Most schemes need an account number only the partner can mint, so creating one is really *recording* correspondence you already hold. The currencies listed here are the ones where that is not true — we issue the reference ourselves and there is a live rail behind it — so they are the only ones a business can open unaided. Computed per request from the live rail table, so a corridor going live or a rail being switched off changes the answer without a deploy. Registered above `/{account_id}` on purpose. FastAPI matches in registration order and `options` is a valid UUID path as far as the router is concerned, so the reverse order sends every call here into the by-id handler and answers 422. See `tests/unit/api/test_route_order.py`.

curl -X GET "https://example.com/api/b/v1/virtual-accounts/options"

{  "currencies": [    {      "currency": "string",      "scheme": "string",      "reference_required": true    }  ]}